Assessment Comments
Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in summers of 2006, 2007, and 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 2007.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to levels of indicator bacteria that exceed Iowa’s water quality standard, and violations of the state water quality criterion for pH. Algae levels that are approaching the impairment level also contribute to the impairment at this lake. The Class B(LW) uses are assessed (monitored) as “partially supported” due to violations of the state water quality criterion for pH. Fish consumption uses are assessed (monitored) as "fully supported" based on results of fish tissue (RAFT) sampling in 2007. Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2006, 2007, and 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (4) information from the IDNR Fisheries Bureau, and (5) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 2007.
Note: A TMDL for nutrients and siltation at Lake MacBride was prepared by IDNR and approved by EPA in 2005. Because, however, the Section 303(d) impairment due to indicator bacteria at this lake was not addressed in the TMDL, this lake is placed in IR Category 5a (impaired; TMDL required).
EXPLANATION: Results of IDNR beach monitoring from 2006 through 2008 suggest that the Class A1 uses be assessed as "fully supported." Levels of indicator bacteria at Lake Macbride beach were monitored once per week during the primary contact recreation seasons (May through September) of 2006 (28 samples), 2007 (15 samples), and 2008 (25 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology, all thirty-day geometric means for the three-year assessment period must be less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses. If a 5-sample, 30-day geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported”. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
At Lake Macbride beach, the geometric means of all 55 thirty-day periods during the summer recreation seasons of 2006, 2007, and 2008 were below the Iowa water quality standard of 126 E. coli orgs/100 ml. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 4% in 2006, 0% in 2007, and 0% in 2008. These results suggest very low levels of bacteria at Lake MacBride beach. However, Lake MacBride was assessed as "not supporting" the Class A1 uses for the 2008 assessment/listing cycle due to exceedences of the geometric mean criterion in 2004. Based on IDNR's assessment/listing methodology two assessment/listing cycles with no exceedences of the geometric mean criterion and without significantly greater than 10% of the samples exceeding the single-sample maximum criterion are necessary to propose de-listing of the bacteria impairment. Therefore, for the 2010 assessment/listing cycle the Class A1 uses will remain assessed as "not supported." If bacteria levels remain low in 2009 and 2010 the bacteria impairment will be suggested for de-listing for the 2012 assessment/listing cycle.
Results from the ISU and UHL lake surveys suggest that the Class A1 (primary contact recreation) uses should be assessed (monitored) as "fully supported/threatened" due to increasing levels of algae (chlorophyll a). Using the median values from these surveys from 2004 through 2008 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 60, 64, and 61 respectively for Lake Macbride. According to Carlson (1977) the Secchi depth value places Lake Macbride at the upper end of the eutrophic category while the chlorophyll a and total phosphorus values place Lake Macbride in between the eutrophic and hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and moderately high levels of phosphorus in the water column. The chlorophyll TSI value has been increasing over the past assessment/listing cycles (2004: 55, 2006: 56, 2008: 62) and now is near the impairment trigger level (65). If this increasing trend continues the chlorophyll TSI value will suggest impairment for the 2012 assessment/listing cycle therefore for the current (2010) assessment listing cycle the algae levels suggest that the Class A1 uses be assessed as "fully supported/threatened."
The level of inorganic suspended solids was low at this lake and does not suggest impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Lake Macbride was 2.0 mg/L, which was the 29th lowest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Lake Macbride, which does not contribute to impairment at this lake. These data show that cyanobacteria comprised 70% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (14.5 mg/L) was also the 63rd highest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to a high frequency of violations of Iowa’s criterion for pH. Based on data from the ISU and UHL lake surveys from 2004 through 2008, there were no violations of the Class B(LW) criterion for ammonia in 26 samples or of the Class B(LW) criterion for dissolved oxygen in 27 samples. There were, however 7 violations of the Class A1,B(LW) criterion for pH in 27 samples (26%). Based on IDNR’s assessment methodology these results are significantly greater than 10% of the samples and therefore constitute an impairment (partial support/monitored) of the Class A1 and B(LW) uses of Lake Macbride.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Lake MacBride in 2007. The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.179 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.0627 ppm. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the 2007 RAFT sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
